Born probably near Spalatum (Split) with the name Diocles on 22 December AD 240 or 245, Diocletian was the son of a poor family in Dalmatia.
It is said, that his father, apparently a scribe of a wealthy senator, might have been a former slave.

Diocles rose through the ranks of the military and achieved high position. Throughout the AD 270's he was military commander in Moesia. From AD 283 onwards, under Carus and his son and successor Numerian he acted as commander of the imperial bodyguard (protectores domestici) and appears a rather dubious figure in the deaths of both of those emperors.

In November AD 284, near Nicomedia he was chosen by the soldiers to avenge Numerian's death, which he did by charging Arrius Aper, the praetorian prefect, whom he sentenced to death. Thereafter he personally executed Aper in front of the troops.

Hailed emperor on 20 November AD 284, immediately, or shortly after this execution, Gaius Aurelius Valerius Diocletian - the name he assumed with the imperial title - crossed the Bosporus into Europe and met the forces of Numerian's brother and co-emperor Carinus at Margum on 1 April AD 285.
Diocletian was in fact losing the battle as the assassination of Carinus by one of his own officers, left the opposing army without a leader. With only one imperial candidate still left on the field, Carinus' army surrendered accepting Diocletian as emperor.
Carinus' murder would also suggest a possible involvement by Diocletian, connecting him (although solely by rumour) with the possible assassination of three emperors.

Seeing it necessary display goodwill to the supporters of Carinus, Diocletian kept Carinus' praetorian prefect, Aristobolus, as well as keeping many of the former emperor's government officials in place.

Then, much to everyone's surprise, Diocletian, in November AD 285 appointed his own comrade Maximian as Caesar and granted him control over the western provinces. Surprising as this development no doubt was, Diocletian urgently needed to give the problems on the Danubian borders his full attention. Meanwhile he needed someone in Rome to take care of government. Not having a son, it was a natural choice to pick one of his trusted military comrades to hold the fort for him.

With Maximian proving himself a worthy Caesar, Diocletian only several months later, on 1 April AD 286, promoted him to the rank of Augustus. Diocletian however remained the senior ruler, possessing a veto over any edicts made by Maximian.

The year Ad 286 however, should not only be remembered for the promotion of Maximian. It also should become known for the rebellion of Carausius, who was the commander of the North Sea fleet, who made himself emperor of Britain.

Meanwhile Diocletian embarked on several years of hard campaigning. Mostly along the Danube frontier, where he defeated German and Sarmatians tribes. One expedition took him as far as Syria, where he campaigned against the Saracen invaders from the Sinai peninsula in AD 290.

Then in AD 293 Diocletian took another huge step into the unknown by founding the 'Tetrarchy', the rule of four. This entirely new idea of imperial government, meant that four emperors should rule the empire. Two Augusti would rule as major emperors, one in teh east, the other in the west. Each Augustus would adopt as his son a junior emperor, a Caesar, who would help rule his half of the empire with him and who be his appointed successor.
The two men who were appointed to these positions were Constantius and Galerius, both military men of Danubian origin.

Had the empire been divded before then Diocletian's division was far more systematic. Each of the tetrarchs had his own capital city, in a territory under his control. The idea was to create a system by which heirs to the throne were appointed by merit and would ruling as Caesars long before the place of Augustus would become vacant. They would then be the automatic heir to the throne and would appoint the next Caesar, by merit.
So in theory at least, this system would assure that the best men for the job, ascended to the throne.
The tetrarchy did not officially split the empire into east and west. It remained one unit, but was ruled by four men.

In AD 296 the Persians attacked the empire. Their successes inspired the revolt of Lucius Domitius Domitianus, after whose death Aurelius Achilleus succeeded as 'emperor' of Egypt. Diocletian moved to put down the revolt and in early AD 298 Achilleus was defeated and killed at Alexandria.
Meanwhile Galerius, the eastern Caesar being groomed to succeed Diocletian, successfully campaigned against the Persians.

Under Diocletian the imperial court was much expanded and elaborated. People were to kneel before their emperor, kissing the hem of his robes. All this wasno doubt introduced to yet further increase the authority of the imperial office. Under Diocletian the emperor became a god-like creature, detached from wordly affairs of the lesser people areound him.
It is considering these intentions that one must regard Diocletian and Maximian declaring themselves the respective sons of Jupiter/Jove and Hercules. This spiritual link between them and the gods, Diocletian adopting the title Jovianus and Maximian the one of Herculianus, was to further elevate them and set them apart from the world around them. No previous emperor had ever gone so far. But it was the pagan equivalent of ruling 'by the will of God', which Christian emperors were to do in years to come.

If Diocletian elevated his own position then he further reduced the power of the provincial governors. He doubled the number of provinces to 100. Controllig only such small areas, it was almost impossible now for a governor to launch a rebellion.

To help oversee this patchwork of little provinces, thirteen dioceses were created, which acted as regional authorities over the provinces. These dioceses were each ruled by a vicarius. In turn, the vicarii were controlled by the four main administrators of the empire, the pratorian prefects (one praetorian prefect per tetrarch).
The administration of government was largely left in the hands of the prefects. They were no longer really military commanders, but far more they were expert jurists and administrators overseeing imperial administration.
Were Diocletian's reforms indeed far-reaching then one of their effects was to significantly reduce the power of the senate. This no doubt will not have been a coincidence.

If Diocletian reformed the way the empire was governed then he did not stop there. First and foremost of the changes was that consription for Roman citizens was reintroduced. The army also was significantly changed in the way it operated. The forces were divided into two parts. One part were the frontier troops guarding the borders, the limitanei, the other, highly mobile forces stationed inland, away from the immediate frontiers, and who could rush to any trouble spot, were the comitantenses.
Further the fleet was expanded.

This expansion of the military under Diocletian represented a large increase compared to the previous reigns. With now well over half a million men under arms, as well as a struggling economy, the tax burden was becoming hard to bear for the ordinary population.
Diocletian's government though was well aware of this. Under his administration a complex taxation system was created which allowed for regional variations of harvests and trade. Areas with more fertile soil or wealthier trade were hence taxed harder than poorer regions.
In AD 301 the Edict of Maximum Prices imposed throughout the empire tried to fix prices and wages in order to curb inflation. The system however did more damage than it did good. Regional price variations no longer existed and therefore trade suffered. Many goods also became unprfitable to sell, which therefore also meant that trade in those goods simply disappeared.

But Diocletian, the great reformer of the empire, should also become known for a very harsh persecution of the Christians. Trying to strengthen Roman traditions, he much revived worship of the old Roman gods. The foreign cults however, Diocletian had no time for.
In AD 297 or 298 all soldiers and administrators were ordered to make sacrifices to the gods. Anyone who refused to do so, was immediately dismissed.

On 24 February AD 303 another edict was issued. This time Diocletian ordered the destruction of all churches and scriptures within the empire. More edicts followed that year, ordering all Christian clergy to be thrown in prison, to be released only after having made sacrifice to the Roman gods.

In April AD 304 Diocletian issued his final religious edict. All Christians were ordered to Roman gods. Anyone who would refuse would be executed.

Then, after a serious illness in AD 304, he took a step - unimaginable to Romans - of abdicating from the throne on 1 May AD 305, compelling a reluctant Maximian to do the same.

From his place of retirement at Spalatum (Split) in Dalmatia, Diocletian briefly returned to the political scene in AD 308 to aid Galerius at the Conference of Carnuntum. After this he withdrew back to Spalatum, where he died on 3 December AD 311.